The present invention relates generally to seismic exploration and more particularly to a method for inversion of seismic data to yield estimates of formation lithology.
In seismic exploration, it is conventional to place a plurality of seismic receivers along the earth's surface at spaced locations. A plurality of seismic sources disposed at spaced locations along the earth's surface can then be activated to generate seismic waves which propagate outwardly in all directions. The seismic waves thus generated are refracted, reflected, and diffracted from subsurface formation interfaces and some of these diverted seismic waves are detected by the seismic receivers and can be processed to form a seismic signal. Such seismic signals can be displayed as seismic sections which contain information about the reflectivity of the subsurface formations to the imparted seismic energy as represented by the time, duration and intensity of reflection events in the seismic signals.
As the field of seismic exploration has advanced, geophysicists have sought additional methods for extracting information about the earth's subsurface formations from seismic reflectivity data. By way of example, Thompson et al. in U.S. Pat. Nos. 4,375,090 and 4,398,273 describe methods for interpreting seismic reflectivity data to yield indications of oil and gas in subsurface formations. Herkenhoff et al. in U.S. Pat. No. 4,554,649 and Ostrander in U.S. Pat. No. 4,562,558 describe methods for interpreting seismic reflectivity data to yield indications of gas-bearing lithologies. Additionally, Sondergeld et al. in U.S. Pat. No. 4,692,910 describe methods for determining formation lithologies from well logging compressional velocity and shear velocity transmission data. Whereas, Thomas et al. in U.S. Pat. No. 4,422,165 describe a method for estimating the ratio of velocities of compressional and shear waves in subsurface formations from seismic reflectivity data. Although these methods each have certain advantages, they are incapable or insufficiently developed to fully exploit the maximum resolution attainable from seismic reflectivity data.
In spite of such advances, the geophysical art has failed to provide a method for the inversion of seismic reflectivity data to yield estimates of formation lithology or to produce lithology sections having a resolution comparable to the resolution of seismic reflectivity data from which they are obtained. The present invention provides a novel method for addressing such shortcomings as well as a novel method for transforming estimates of formation lithology into chromatic lithology sections having unique colors assigned to selected combinations of formation lithologies.